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Sinking rates responses of oceanic phytoplankton to irradiance, nutrients, and iron stress Lecourt, Maude


Emiliania huxleyi (Lohmann), a small oceanic coccolithophore, was isolated from the NE Subarctic Pacific Ocean. The sinking rates of two strains of E. huxleyi, naked and coccolith-forming, were measured. The two strains were grown under saturating and lightlimited conditions in media containing either NO3 or NH4 as the primary nitrogen source. Sinking rates were measured during log and senescent growth phases. The naked strain grew significantly faster under saturating light than the coccolith-forming strain. The coccolith-forming strain of E. huxleyi had a significantly larger cell volume and higher sinking rates than the naked strain under all conditions. The naked strain increased significantly its chi a content under light limitation. POC and PUN were significantly higher for the coccolith-forming strain. During senescence, coccolith formation and cell aggregation increased for the coccolith-forming strain and the naked strain, respectively, significantly increasing their sinking rates. No significant difference in physiological parameters was observed between NO3 and NH4+ grown cells, except that growth rate was faster under NH4 at saturating light. These results suggest that coccolith formation controls sinking rates, and therefore the coccolith-forming strain has some advantages over the naked strain. This study also determined the sinking rates responses, physiological parameters, and iron quotas of E. huxleyi (coccolith forming strain) and an oceanic strain of the diatom Actinocyclus sp. (20 im diameter) under iron deficiency. Actinocyclus sp. was grown under N03- and N4+H to determine whether there was an advantage for cells to grow on NH4 rather than NO3- under iron-deplete conditions. E. huxleyi was grown on NO3- only. Under iron-deficient conditions, Actinocyclus sp. increased its sinking rate 9 fold, despite its 30% decrease in cell volume. E. huxleyi maintained its sinking rates, but deceased its cell volume by 50%. Growth rates and iron quotas of Actinocyclus sp. and E. huxleyi significantly decreased under iron-deplete conditions. E. huxleyi maintained its POC and PUN and increased its chi a content under iron-deplete conditions. On the other hand, Actinocyclus sp. significantly decreased its POC, PON and its chi a content when grown on N03. The results obtained under iron-deplete conditions indicate that E. huxleyi grown on N0 3 was not significantly affected by iron stress. However, Actinocyclus sp. was affected by iron deficiency, and was energetically affected by the nitrogen source. Finally, small coccolithophores are better adapted to their habitat (i.e. low iron concentrations in the NE Subarctic Pacific) and can easily outcompete large diatoms in a low iron environment.

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